Electric power steering apparatus

ABSTRACT

An electric power steering apparatus includes a speed reducing mechanism for decelerating the rotation of a rotatable shaft in an electric motor and a converting mechanism for converting the output rotation of the speed reducing mechanism into the axial movement of a steering shaft extending toward the right and left sides of a vehicle. The speed reducing mechanism includes an input pulley driven by the electric motor, an output pulley arranged with the steering shaft enclosed thereby, a belt for connecting the input pulley and the output pulley to each other, and a belt tension adjuster.

This is a Divisional of U.S. Application Ser. No.: 10/352,249, filedJan. 28, 2003, now U.S. Pat. No. 6,938,722 the subject matter of whichis incorporated entirely herein by reference.

CROSS REFERENCE TO RELATED APPLICATIONS

The present invention corresponds to and claims benefit of applicationsNo. 2002-020347 and No. 2002-020346 filed with the Japan Patent Officeon Jan. 29, 2002, an application No. 2002-040327 filed with the JapanPatent Office on Feb. 18, 2002, and an application No. 2002-044911 filedwith the Japan Patent Office on Feb. 21, 2002, all the disclosures ofwhich are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electronic power steering apparatusthat generates a steering assist force by an electric motor.

2. Description of Related Arts

In a rack assist type electronic power steering apparatus, the rotationof an electric motor is decelerated through a pulley/belt mechanism, andis then converted into the axial movement of a rack shaft through ascrew ball mechanism enclosing a rack shaft (see JP-B-4-28583, forexample).

In this case, a belt is interposed in a power transmission path betweenthe electric motor and the rack shaft. Accordingly, a shock load andvibration from the rack shaft are not transmitted to the electric motor.

However, vibration may be generated in the belt due to fluctuations inthe tension of the belt, and transmitted to a housing to propagate intoa vehicle chamber, to be noise.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an electric powersteering apparatus capable of reducing vibration and noise.

The present invention relates to an electric power steering apparatuscomprising an electric motor for producing a steering assist force. Theapparatus further comprises a speed reducing mechanism for deceleratingthe rotation of a rotatable shaft in the electric motor; and aconverting mechanism for converting the output rotation of the speedreducing mechanism into the axial movement of a steering shaft extendingtoward the right and left sides of a vehicle. The speed reducingmechanism comprises an input pulley driven by the electric motor, anoutput pulley arranged with the steering shaft enclosed thereby, a beltfor connecting the input pulley and the output pulley, and a tensionadjusting mechanism for adjusting the tension of the belt.

According to the present invention, the tension of the belt is properlyadjusted, thereby making it possible to prevent inferior operation andnoise generation caused by too much slack in the belt.

The tension adjusting mechanism may include a center-to-center distancechanging mechanism for chancing a center-to-center distance between theinput pulley and the output pulley. The tension adjusting mechanism maycomprise a fixed member, a movable member, a tensioner pulley supportedon the movable member so as to be rotatable and engaging with the belt,and urging means for urging the tensioner pulley through the movablemember in the direction in which tension is applied to the belt.

It is preferable that a supporting shaft in the input pulley iselastically supported on a housing.

It is preferable that a pair of input pulleys, a pair of output pulleys,and a pair of belts are respectively provided, and the input pulley andthe output pulley which correspond to each other are connected to eachother by a corresponding belt.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing the schematic configuration of anelectric power steering apparatus according to a first embodiment of thepresent invention;

FIG. 2 is a cross-sectional view of a principal part of the electricpower steering apparatus according to the first embodiment;

FIG. 3 is a cross-sectional view taken along a line III—III shown inFIG. 2;

FIG. 4 is a cross-sectional view of a principal part of an electricpower steering apparatus according to a second embodiment of the presentinvention;

FIG. 5 is a cross-sectional view of a principal part of an electricpower steering apparatus according to a third embodiment of the presentinvention;

FIG. 6 is a cross-sectional view of a principal part of an electricpower steering apparatus according to a fourth embodiment of the presentinvention;

FIG. 7 is a cross-sectional view of a principal part of an electricpower steering apparatus according to a fifth embodiment of the presentinvention;

FIG. 8 is a cross-sectional view taken along a line VIII—VIII shown inFIG. 7;

FIG. 9 is a cross-sectional view showing a sixth embodiment of thepresent invention;

FIG. 10 is a cross-sectional view of a principal part of an electricpower steering apparatus according to a seventh embodiment of thepresent invention;

FIG. 11 is a cross-sectional view taken along a line XI—XI shown in FIG.10;

FIG. 12 is an enlarged view of a principal part of an input pulley and abelt in the seventh embodiment;

FIG. 13 is a cross-sectional view of a principal part of an electricpower steering apparatus according to an eighth embodiment of thepresent invention;

FIG. 14 is an enlarged view of a supporting structure of an input pulleyand an input shaft in the eighth embodiment;

FIG. 15 is a cross-sectional view of a principal part of an electricpower steering apparatus according to a ninth embodiment of the presentinvention;

FIG. 16 is an enlarged view of a principal part of an input pulley and abelt in the ninth embodiment;

FIG. 17 is a schematic view showing a phase shift between a pair ofinput pulleys in the ninth embodiment; and

FIG. 18 is a cross-sectional view of a principal part of an electricpower steering apparatus according to a tenth embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Preferred embodiments of the present invention will be described whilereferring to the drawings.

First Embodiment

FIG. 1 is a schematic view showing the schematic configuration of anelectric power steering apparatus according to a first embodiment of thepresent invention. Referring to FIG. 1, an electric power steeringapparatus (EPS) 1 comprises a steering shaft 3 connected to a steeringwheel 2 serving as a steering member, and a rack shaft 6 serving as asteering shaft having a pinion gear 4 provided at a front end of thesteering shaft 3 and a rack gear 5 engaging with the pinion gear 4 andextending toward the right and left sides of a vehicle.

Tie rods 7 are respectively coupled to both ends of the rack shaft 6.Each of the tie rods 7 is connected to a corresponding wheel 8 through acorresponding knuckle arm (not shown) When the steering wheel 2 isoperated so that the steering shaft 3 is rotated, the rotation isconverted into the linear motion of the rack shaft 6 along the right andleft sides of the vehicle by the pinion gear 4 and the rack gear 5.Consequently, the rolling steering of the wheel 8 is achieved.

The steering shaft 3 is divided into an input shaft 9 connecting withthe steering wheel 2 and an output shaft 10 connecting with the piniongear 4. The input and output shafts 9 and 10 are coaxially connected toeach other through a torsion bar 11.

There is provided a torque sensor 12 for detecting a steering torque bythe amount of displacement of the relative rotation between the inputshaft 9 and the output shaft 10 through the torsion bar 11. The resultsof the detection of the torque by the torque sensor 12 are given to acontrol section 13. In the control section 13, a voltage to be appliedto an electric motor 15 for assisting steering through a driver 14 iscontrolled on the basis of the results of the detection of the torque,the results of the detection of the vehicle speed, etc. The rotation ofa rotatable shaft 16 (see FIG. 2) in the electric motor 15 isdecelerated through a speed reducing mechanism 17 including apulley/belt mechanism. The output rotation of the speed reducingmechanism 17 is converted into the axial movement of the rack shaft 6through a converting mechanism 18, so that steering is assisted. Theelectric power steering apparatus 1 is of a so-called rack assist type.

Then, FIG. 2 is an enlarged view of a principal part of the electricpower steering apparatus 1, and FIG. 3 is a schematic cross-sectionalview along a line III—III shown in FIG. 2.

Referring to FIGS. 2 and 3, a motor housing 20 in the electric motor 15is fixed to a second housing 19 through a first housing 21. The electricmotor 15 is provided side by side with the second housing 19. The firsthousing 21 holds an input pulley 28, described later, in the speedreducing mechanism 17. The second housing 19 functions as a rack housingaccommodating the rack shaft 6.

The first housing 21 has a cylindrical shape as a whole, and is providedwith mounting flanges 22 serving as a pair of opposite portions to havean approximately ▪ shape in cross section. The second housing 19 hasmounting flanges 23 serving as opposite portions which are respectivelyopposed to the mounting flanges 22 in the first housing 21. Both themounting flanges 22 and 23 are fastened to each other by a screw 25 in astate where a spacer 24 such as a shim is interposed between themounting flanges 22 and 23. The screw 25 is inserted through a screwinsertion hole 22 a of the mounting flange 22, for example, and isscrewed into a screw hole 23 a of the mounting flange 23.

The present embodiment is characterized in that the distance between thefirst housing 21 and the second housing 19 is adjusted by adjusting thethickness of the spacer 24, thereby adjusting the distance betweencenter axes 28 a and 29 a of the input and output pulleys 28 and 29 (acenter-to-center distance D), as described later, through which theadjustment of the tension of a belt 30 is achieved. That is, acenter-to-center distance changing member serving as a tension adjustingmember is constituted by the spacer 24.

The speed reducing mechanism 17 comprises an input shaft 27 coaxiallyconnected to the rotatable shaft 16 in the electric motor 15 through acoupling joint 26 using a spline 31 a, for example, an input pulley 28having a small diameter provided so as to be integrally rotatable in anintermediate portion in the axial direction of the input shaft 27, anoutput pulley 29 having a large diameter arranged with the rack shaft 6serving as a steering shaft enclosed thereby, and a belt 30 wrappedbetween the input and output pulleys 28 and 29. The belt 30 is composedof a toothed belt (cogged belt), for example, and the input pulley 28 isconstructed as a toothed pulley having teeth engaging with the toothedbelt formed at equally spaced divisions in its circumferential directionon the outer periphery of the input shaft 27. A toothed pulley is alsosimilarly used as the output pulley 29.

The input shaft 27 has first and second ends 31 and 32, and the firstand second ends 31 and 32 are supported so as to be rotatable bycorresponding supporting holes 35 and 36 in the first housing 21 throughcorresponding bearings 33 and 34.

The first end 31 of the input shaft 27 is connected so as to beintegrally rotatable to the rotatable shaft 16 in the electric motor 15through the coupling joint 26.

The first housing 21 comprises a connection housing 37 connected to themotor housing 20 so as to cover a portion where the rotatable shaft 16projects from the motor housing 20, and a speed reducing mechanismhousing 39 for defining an accommodation chamber 38 accommodating aprincipal part of the speed reducing mechanism 17 in corporation withthe connection housing 37.

The connection housing 37 has a cylindrical shape, and accommodates theabove-mentioned coupling joint 26. The connection housing 37 has aperipheral wall 40 and an end wall 41, and the end wall 41 is providedwith the above-mentioned supporting hole 35.

The speed reducing mechanism housing 39 has a peripheral wall 42 fittedin the peripheral wall 40 of the connection housing 37 in a fluid-tightmanner, and an end wall 43. The end wall 43 is provided with theabove-mentioned supporting hole 36. The input pulley 28 is accommodatedin the accommodation chamber 38 defined by the connection housing 37 andthe speed reducing mechanism housing 39. The above-mentioned mountingflanges 22 are provided in both the connection housing 37 and the speedreducing mechanism housing 39.

As a conversion mechanism 18, rotational motion can be converted intolinear motion using a ball screwmechanism or a bearing screw mechanism,for example (see JP-A-2000-46136, for example). In the presentembodiment, description is made in conformity with an example in whichthe ball screw mechanism is used. The converting mechanism 18 comprisesa ball nut 44 serving as a body of rotation surrounding the rack shaft6.

The ball nut 44 is screwed into a ball screw groove 6 a formed in ahalfway portion of the rack shaft 6 through a ball screw 45, therebyconstituting the converting mechanism 18. The ball nut 44 is supportedon the rack housing 19 so as to be rotatable through a bearing 46.Further, the above-mentioned output pulley 29 is fitted in an outerperiphery 47 of the ball nut 44 so as to be integrally rotatable.Specifically, the output pulley 29 is fixed to the ball nut 44 byholding the output pulley 29 between a, step 48 formed in the outerperiphery 47 of the ball nut 44 and a fixed screw 50 screwed into ascrew 49 in the outer periphery 47.

According to the present embodiment, the distance between the centeraxes 28 a and 29 a of the input and output pulleys 28 and 29 (thecenter-to-center distance D) can be adjusted easily and at low cost byadjusting the thickness of the spacer 24 serving as a center-to-centerdistance changing member composed of a shim or the like. Furthermore,the tension of the belt 30 can be adjusted easily and at low cost, andinferior operation and noise generation related to the belt 30 can beprevented.

Second Embodiment

Then, FIG. 4 illustrates a second embodiment of the present invention.Referring to FIG. 4, the present embodiment differs from the firstembodiment shown in FIG. 3 in that the spacer 24 is abandoned, and afirst housing 21 for holding an input pulley 28 and a rack housing 19serving as a second housing for holding an output pulley 29 through abearing 46 and a rack shaft 6 are relatively slid in a lateral directionX along their mounting flanges 22 and 23 to adjust a center-to-centerdistance D between respective center axes 28 a and 29 a of the input andoutput pulleys 23 a and 29 a. The lateral direction X is a directionapproximately perpendicular to a plane P including the center axes 28 aand 29 a.

In order to allow the foregoing, a screw insertion hole 22 b of themounting flange 22 is formed into a slot along the lateral direction X,to guide the relative slide between both the housings 19 and 21 in thelateral direction X through the screw insertion hole 22 b composed ofthe slot. The second embodiment is the same as the first embodimentshown in FIG. 3 in that a screw hole 23 a screwed into a screw 25 isformed in the mounting flange 23. The mounting flanges 22 and 23 servingas opposite portions which are brought into contact with each other, thescrew 25, and the screw insertion hole 22 b composed of the slotconstitute a guiding mechanism G1 serving as a center-to-center distancechanging mechanism.

In the second embodiment, both the housings 21 and 19 are relativelyslid in the lateral direction X, thereby making it possible to adjustthe tension of the belt 30 easily and at low cost while achieving spacesaving.

Third Embodiment

Then, FIG. 5 illustrates a third embodiment of the present invention.Referring to FIG. 5, the third embodiment differs from the secondembodiment shown in FIG. 4 in that both the housings 21 and 19 arerelatively slid along a lateral direction X in the second embodimentshown in FIG. 4, while a rack housing 19 and a connection housing 37Aare relatively slid along a longitudinal direction Y to adjust acenter-to-center distance D, and there is provided a single drive screw51 for relatively sliding the rack housing 19 and the connection housing37A along the longitudinal direction Y in the present embodiment. Thelongitudinal direction Y is a direction approximately perpendicular toboth a center axis 28 a and a center axis 29 a.

Specifically, there is provided a guided section 52 composed of anoutward annular flange in a peripheral wall 40 of the connection housing37A, and there is provided an extended section 53 extending parallel toan input shaft 27 from an end wall 41 of the connection housing 37A.Further, an end wall 54 parallel to an end wall 43 of a speed reducingmechanism housing 39A is provided so as to extend from a front end ofthe extended section 53, and a cylindrical section 55 projecting in astepped shape is formed in the end wall 54. An end surface of thecylindrical section 55 forms a guided section 56. A second end 32 of theinput shaft 27 is supported so as to be rotatable by an inner peripheralsurface 55 a of the cylindrical section 55 through a bearing 34.

On the other hand, the speed reducing mechanism housing 39A isintegrally formed as a part of the rack housing 19, and a guidingsection 57 for guiding the above-mentioned guided section 52 is formedin the rack housing 19 and the speed reducing mechanism housing 39A.Further, a guiding section 58 composed of a recess, for example, forguiding the guided section 56 is formed in the end wall 43 of the speedreducing mechanism housing 39A. The guided sections 52 and 56 and theguiding sections 57 and 58 constitute a guiding mechanism G2. The drivescrew 51 is inserted through a screw insertion hole 59 of a peripheralwall 42 of the speed reducing mechanism housing 39A, and is screwed intoa screw hole 60 of the extended section 53 in the connection housing37A. The guiding mechanism G2 and the drive screw 51 constitute acenter-to-center distance changing mechanism A1.

According to the present embodiment, the tension of a belt 30 can beadjusted easily and with high precision by managing a torque forfastening the drive screw 51, and inferior operation and noisegeneration related to the belt 30 can be prevented.

Fourth Embodiment

Then, FIG. 6 illustrates a fourth embodiment of the present invention.Referring to FIG. 6, the present embodiment differs from the embodimentshown in FIG. 5 in that the center-to-center distance D is increased bypulling the connection housing 37A for holding an input pulley 28 andthe motor housing 20 in the operation of the single drive screw 51, toincrease the tension of the belt 30 in the embodiment shown in FIG. 5,while a center-to-center distance D is increased by pressing aconnection housing 37B for supporting a first end 31 of an input shaft27 and a support housing 61 for supporting a second end 32 of the inputshaft 27 along a longitudinal direction Y in the operation of a pair ofdrive screws 51 a and 51 b arranged on both sides with a belt 30interposed therebetween, to increase the tension of the belt 30 in thepresent embodiment.

The connection housing 37B has a reduced diameter portion 62 and a largediameter portion 63 nearer to its front end than the reduced diameterportion 62, to support the first end 31 of the input shaft 27 through abearing 33 by the large diameter portion 63. The large diameter portion63 has a guided section 64 composed of a pair of annular flanges.

The support housing 61 has the shape of a cylinder having a bottom, tosupport the second end 32 of the input shaft 27 through a bearing 34.The bottom of the support housing 61 and the annular flanges constitutea pair of guided sections 65.

On the other hand, a speed reducing mechanism housing 39B is integrallyformed in a rack housing 19, and has a pair of holding sections 66 and67 having a groove shape in cross section for respectively holding theconnection housing 37B and the support housing 61. A pair of oppositewalls of the holding sections 66 and 67 constitutes guiding sections 68and 69 for respectively guiding the guided sections 64 and 65. Theguided sections 64 and 65 and the guiding sections 68 and 69 for guidingthe guided sections 64 and 65 constitute a guiding mechanism G3, and theguiding mechanism G3 and a pair of drive screws 51 a and 51 b constitutea center-to-center distance adjusting mechanism A2 serving as a tensionadjusting mechanism.

The pair of drive screws 51 a and 51 b is screwed into screw holes 70respectively formed in the holding sections 66 and 67, to abut theirfront ends against the large diameter portion 63 in the connectionhousing 37B and an outer peripheral surface of the support housing 61.

According to the present embodiment, the input shaft 27 and an inputpulley 28 can be moved with high precision parallel to a center axis 29a of an output pulley 29 by the pair of drive screws 51 a and 51 b onboth sides with the belt 30 interposed therebetween. In the presentembodiment, the center-to-center distance D may be adjusted by pullingthe connection housing 37B and the support housing 61 upward in FIG. 6by the pair of drive screws 51 a and 51 b.

Fifth Embodiment

Then, FIGS. 7 and 8 illustrate a fifth embodiment of the presentinvention. Referring to FIGS. 7 and 8, the present embodiment ischaracterized in the following. That is, a fixed housing 39C serving asa speed reducing mechanism housing integrally formed in a rack housing19 is provided with a circular hole 71. A movable housing 72 integrallyextending from a motor housing 20 in an electric motor 15 has a circularradially outer portion 73 fitted in the circular hole 71 so as to berotatable. Further, the movable housing 72 has an eccentric hole 74 forrespectively supporting first and second ends 31 and 32 of an inputshaft 27 so as to be rotatable through bearings 33 and 34. The center 74a of the eccentric hole 74 is eccentric from the center 73 a of thecircular radially outer portion 73 in the movable housing 72(corresponding to the center 71 a of the circular hole 71).

A center-to-center distance adjusting mechanism A3 serving as a tensionadjusting mechanism, including the circular hole 71, the movable housing72, and the eccentric hole 74, is constructed. Further, either one offlanges 75 and 76 against which the fixed housing 39C and the movablehousing 72 are abutted is provided with a screw hole 77, and the otherflange is provided with a screw insertion hole 78 in a circular arcshape. A fixed screw 79 through which the screw insertion hole 78 in acircular arc shape is inserted is screwed into the screw hole 77, tofasten the flanges 75 and 76 in both the housings 39C and 72 to eachother such that the rotational position is adjustable.

According to the present embodiment, a center-to-center distance Dbetween a center axis 28 a of an input pulley 28 and a center axis 29 aof an output pulley 29 is adjusted only by rotating the movable housing72, together with the motor housing 20 in the electric motor 15, to movethe center axis 28 a farther away from the center axis 29 a, therebymaking it possible to adjust the tension of a belt 30. The movablehousing 72 and the motor housing 20 can be also separately constructed.

Sixth Embodiment

Then, FIG. 9 illustrates a sixth embodiment of the present invention.Referring to FIG. 9, a center-to-center distance changing mechanism A4in the present embodiment differs from the center-to-center distancechanging mechanism A2 in the embodiment shown in FIG. 6 in that a pairof drive screws 51 a and 51 b is replaced with the pair of elasticmembers 80 and 81 composed of a compression coil spring, for example.

In the sixth embodiment, a connection housing 37B and a support housing61 for respectively supporting first and second ends 31 and 32 of aninput shaft 27 are elastically urged along a longitudinal direction Y bythe elastic members 80 and 81, thereby making it possible toautomatically adjust the tension of a belt 30 to a proper value for along time.

The present invention is not limited to each of the above-mentionedembodiments. For example, in each of the embodiments, the input andoutput pulleys 28 and 29 may be formed of synthetic resin such aspolyacetal resin to reduce shock (an excitation force) caused byengagement with the belt 30, thereby further reducing noise.

Seventh Embodiment

FIGS. 10, 11, and 12 illustrate a seventh embodiment of the presentinvention.

Although in FIG. 10, an electric power steering apparatus according tothe seventh embodiment is approximately the same in configuration as theelectric power steering apparatus shown in FIG. 2, it differs therefromin that the spacer 24 is abandoned. In FIG. 10, the same components asthose in the embodiment shown in FIG. 2 are assigned the same referencenumerals and hence, the description thereof is not repeated.

Referring to FIG. 11, a tensioner T for adjusting the tension of a belt30 is provided. The tensioner T comprises a tensioner pulley 82 forpressing a slack side L of the belt 30 toward a tension side H. Thetensioner pulley 82 is supported on one end of a swing arm 83 serving asa movable member so as to be rotatable through a supporting shaft 84.The other end of the swing arm 83 is supported on a supporting bracket85 serving as a fixed member fixed to a speed reducing mechanism housing39 through a supporting shaft 86.

The swing arm 83 is rotated and urged in the direction in which tensioncan be applied to the belt 30 (in a counterclockwise direction in FIG.11) by an urging member 87 composed of a torsion coil spring, forexample. The urging member 87 can use a torsion spring wrapped aroundthe supporting shaft 86, for example, as schematically illustrated inFIG. 11. A belt wrap contact area R on an input pulley 28 having a smalldiameter is widened by the function of the tensioner T.

The belt 30 is composed of a toothed belt (cogged belt), for example, asshown in FIG. 12, and the input pulley 28 is constructed as a toothedpulley having teeth 88 engaging with the toothed belt formed at equallyspaced divisions in its circumferential direction on the outer peripheryof an input shaft 27. A toothed pulley is also similarly used as anoutput pulley 29 having a large diameter. In FIG. 12, POD indicates thepitch circle diameter of the input pulley 28 having a small diameter.

According to the seventh embodiment, the belt wrap contact area R on theinput pulley 28 having a small diameter expands by pressing the slackside L of the belt 30 toward the tension side H by the tensioner pulley82. Even when a pulley 28 of a small size is used as the input pulley 28so as to ensure a high reduction gear ratio, therefore, stable torquetransmission can be achieved, and durability can be improved. Byachieving the high reduction gear ratio, it is possible to employ anelectric motor having a high speed and a low torque and also contributeto cost reduction.

Particularly when a toothed pulley engaging with the belt 30 composed ofthe cogged belt is used as the input pulley 28 having a small diameter,as in the present embodiment, the number of teeth 88 engaging with thebelt 30 can be increased in the input pulley 28 having a small diameter,thereby making it possible to increase the effect of improvingdurability.

When the pitch circle diameter PCD of the input pulley 28 (see FIG. 12)is within a range of 12 to 30 mm, it is preferable that a center anglecorresponding to the belt wrap contact area R on the input pulley 28 iswithin a range of 135 to 210 degrees. Consequently, it is possible toensure a sufficient amount of belt wrapping on the input pulley 28having a small diameter and therefore, to ensure stable torquetransmission and durability while achieving a small size and a highreduction gear ratio.

Eighth Embodiment

FIG. 13 and FIG. 14 which is an enlarged view illustrate an eighthembodiment of the present invention. Referring to FIG. 13, a holdinghousing 39D serving as a speed reducing mechanism housing is integrallyformed in a predetermined portion of a rack housing 19.

First and second ends 31 and 32 of an input shaft 27 are respectivelysupported so as to be rotatable on corresponding supporting holes 35 and36 of a connection housing 37 and the holding housing 39D throughcorresponding bearings 33 and 34.

Referring to FIG. 14, a peripheral groove 89 is formed in each of thesupporting holes 35 and 36, and a cushioning material 90A serving aselastic supporting means is accommodated in each of the peripheralgrooves 89. The cushioning materials 90A are respectively interposedbetween the corresponding supporting holes 35 and 36 and outer rings 91of the corresponding bearings 33 and 34, to elastically support thecorresponding outer rings 91. The supporting hole 35 or 36 and the outerring 91 are loosely fitted to each other.

Furthermore, peripheral grooves 93 are respectively formed in an outerperiphery 27 a of the input shaft 27 corresponding to inner rings 92 ofthe bearings 33 and 34, and a cushioning material 90B serving as elasticsupporting means is accommodated in each of the peripheral grooves 93.The cushioning materials 90B respectively elastically support thecorresponding ends 31 and 32 of the input shaft 27. As a result, theinput shaft 27 is elastically supported on the holding housing 39D andthe connection housing 37 through the cushioning materials 90A and 90B.The input shaft 27 and the inner ring 92 are loosely fitted to eachother.

An elastic member such as synthetic rubber or synthetic resin can beused as the cushioning materials 90A and 90B. If an O-ring is used as anannular elastic member, for example, however, standard components can bealso employed, resulting in reduced cost.

According to the eighth embodiment, vibrations respectively transmittedto the holding housing 39D and the connection housing 37 from the inputshaft 27 can be significantly restrained by the functions of thecushioning materials 90A and 90B, thereby making it possible tosignificantly reduce noise within a vehicle chamber.

Although in the eighth embodiment, both the cushioning material 90Adirectly receiving the outer ring 91 and the cushioning material 90Bdirectly receiving the input shaft 27 are provided, the presentinvention is not limited to the same. For example, either one of thecushioning material 90A and the cushioning material 90B may be provided.

Furthermore, a cogged belt may not be employed as a belt.

Ninth Embodiment

FIG. 15 is an enlarged view of a principal part of an electric powersteering apparatus 1. Referring to FIG. 15, the ninth embodiment is thesame as the embodiment shown in FIG. 13 in that a motor housing 20 in anelectric motor 15 is fixed to a holding housing 39D in a rack housing 19through a connection housing 37.

In the ninth embodiment, a speed reducing mechanism 17 comprises aninput shaft 27 coaxially connected to a rotatable shaft 16 in theelectric motor 15 so as to be integrally rotatable through a couplingjoint 26 using a spline 31 a, for example, a pair of input pulleys 28Aand 28B provided in the input shaft 27 so as to be integrally rotatable,a pair of output pulleys 29A and 29B having a large diameter arrangedwith a rack shaft 6 serving as a steering shaft enclosed thereby, and apair of endless belts 30A and 30B respectively wrapped between the inputpulleys 28A and 28B and the output pulleys 29A and 29B whichrespectively correspond to each other.

The belt 30A is composed of a toothed belt (a cogged belt), for example,as shown in FIG. 16, and the corresponding input pulley 28A isconstructed as a toothed pulley having teeth 88 engaging with thetoothed belt formed at equally spaced divisions in its circumferentialdirection on its outer periphery. The belt 30B is also composed of atoothed belt, and the corresponding input pulley 28B is also constructedas a toothed pulley. Further, a toothed pulley is also similarly used asthe output pulleys 29A and 29B, which is not illustrated. The PCDindicates the pitch circle diameter of each of the input pulleys 28A and28B.

Referring to FIG. 17 which is a schematic view, the respectiverotational phases of the pair of input pulleys 28A and 28B are shiftedfrom each other so that the teeth 88 of one of the input pulleys 28A and28B are put at positions corresponding to tooth grooves 94 of the otherpulley.

According to the present embodiment, a torque is transmitted using thepair of belts 30A and 30B in parallel in a torque transmission path,thereby making it possible to reduce a torque applied to each of thebelts 30A and 30B by half, as compared with that in a conventional casewhere it is transmitted by a single belt. As a result, belt vibrationand noise caused thereby can be significantly reduced. Particularly,fluctuations in the tension of the belt in a case where a steering wheelis cut are reduced, thereby making it possible to reduce a warming soundof the belt.

Moreover, the respective phases of the pair of input pulleys 28A and 28Bcomposed of the toothed pulley are shifted from each other such that theteeth 88 of one of the input pulleys 28A and 28B correspond to thepositions of the tooth grooves 94 of the other input pulley.Accordingly, the crest of vibration of one of the belts 30A (30B) isoverlapped with the trough of vibration of the other belt 30B (30A) tocancel both the vibrations. Therefore, fluctuations in the torque can befurther reduced, to further reduce vibration and noise as a whole.Further, the fluctuations in the tension in the case where the steeringwheel is cut can be canceled, thereby making it possible tosignificantly reduce the warming sound.

Tenth Embodiment

Then, FIG. 18 illustrates a tenth embodiment of the present invention.Referring to FIG. 18, the present embodiment is characterized in that apair of first one way clutches 95A and 95B is interposed between aninput shaft 27 and input pulleys 28A and 28B, respectively, and a pairof second one way clutches 96A and 96B is interposed between outputpulleys 29A and 29B and a ball nut 44 serving as an input section of aconverting mechanism 18, respectively. As the one way clutches 95A, 95B,96A, and 96B, various types of known one way clutches can be used inaddition to a sprag type one way clutch, for example.

The first and second one way clutches 95A and 96A corresponding to theone belt 30A and first and second one way clutches 95B and 96Bcorresponding to the other belt 30B are respectively allowed to rotatein opposite directions.

According to the present embodiment, in cutting a steering wheel 2, whena rotatable shaft 16 in an electric motor 15 starts to rotate in theopposite direction, the one belt 30A which has so far rotated in thedirection of rotation of the rotatable shaft 16 starts to idle by thecorresponding first and second one way clutches 95A and 96A, and theother belt 30B which has so far idled starts to rotate in the directionof rotation of the rotatable shaft 16 through the corresponding firstand second one way clutches 95B and 96B, for example, to transmit atorque. As a result, a warming sound at the time of cutting the steeringwheel 2 can be solved. Further, the belt which has so far idledfunctions immediately when the steering wheel 2 is cut. Accordingly,there is no time lag in belt transmission at the time of cutting thesteering wheel 2, thereby making it possible to improve responsibility.

In the tenth embodiment, a cogged belt may not be employed as the belt.

The present invention is not limited to each of the above-mentionedembodiments. For example, a bearing screw mechanism can be used in placeof the ball screw mechanism. Further, the rotatable shaft 16 in theelectric motor 15 and the input shaft 27 can be integrally formed.

While the invention has been described in detail with respect tospecific embodiments thereof, it will be appreciated that those skilledin the art, upon attaining an understanding of the foregoing, mayreadily conceive of alterations to, variations of, and equivalents tothese embodiments. Accordingly, the scope of the present inventionshould be assessed as that of the appended claims and any equivalentsthereto.

1. An electric power steering apparatus, comprising an electric motorfor producing a steering assist force, further comprising: a speedreducing mechanism for decelerating a rotation of a rotatable shaft inthe electric motor; and a converting mechanism for converting an outputrotation of the speed reducing mechanism into an axial movement of asteering shaft extending toward right and left sides of a vehicle, thespeed reducing mechanism comprising a pair of input pulleys disposedcoaxially with the rotatable shaft in the electric motor, a pair ofoutput pulleys arranged with the steering shaft enclosed thereby, and apair of belts for connecting the input pulleys and the output pulleyswhich respectively correspond to each other, wherein each of the pulleysis composed of a toothed pulley, each of the belts is composed of acogged belt, and a phase of teeth of one of the pair of input pulleys isopposite to a phase of teeth of the other input pulley.